Method for effectively transmitting control signal in wireless communication system

ABSTRACT

A method of performing HARQ performed by a user equipment (UE) is provided. The method includes receiving a bundling indicator which indicates the number of bundled downlink subframes, determining whether at least one bundled downlink subframe is missed by comparing the bundling indicator with the number of detected bundled downlink subframes, generating a representative ACK/NACK signal when no bundled downlink subframe is missed, and transmitting the representative ACK/NACK signal on an uplink channel. Recovery capability is maximized and the packet loss is reduced in such a situation that less number of ACK/NACK signals are fed back than that of downlink packets.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/563,472 filed Jul. 31, 2012, which is a continuation of U.S.patent application Ser. No. 13/345,532 (now U.S. Pat. No. 8,250,443,issued on Aug. 21, 2012) filed Jan. 6, 2012, which is a continuation ofU.S. patent application Ser. No. 12/836,403 (now U.S. Pat. No.8,108,757, issued on Jan. 31, 2012) filed Jul. 14, 2010, which is acontinuation of U.S. patent application Ser. No. 12/404,873 (now U.S.Pat. No. 7,774,686, issued on Aug. 10, 2010) filed Mar. 16, 2009, whichclaims the benefit of priority from U.S. Provisional Application No.61/036,985 filed on Mar. 16, 2008, U.S. Provisional Application No.61/047,107 filed on Apr. 23, 2008, and Korean Patent Application No.10-2009-0021715 filed on Mar. 13, 2009. The entire contents of all ofthe above applications are hereby incorporated by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a method for effectively transmitting a control signalin a wireless communication system.

2. Related Art

Next-generation mobile communication systems are being standardized forthe purpose of efficient connection with wired and wirelesscommunication networks and integrated service although communicationsystems of previous generations provide simple wireless communicationservices. As high-speed large-capacity communication systems capable ofprocessing and transmitting information of various types such as videodata and radio data as well as audio data are required, development of atechnique capable of transmitting a large quantity of data, whichcorresponds to the capacity of a wired communication network, through awireless communication network is needed. Accordingly, an appropriateerror detection scheme which can minimize information loss and increasesystem transmission efficiency to improve system performance becomes anessential element.

Automatic Repeat request (ARQ) transmits positive acknowledgement (ACK)signal to a transmitter when a receiver properly receives data andtransmits a negative acknowledgement (NACK) signal to the transmitterwhen the receiver does not properly receive the data. In HybridAutomatic Repeat request (HARQ), an ACK/NACK signal transmitted by adata receiver is represented by a small number of bits, in general.

To improve data transmission efficiency in data processing, HARQcorresponding to a combination of ARQ and channel coding of a physicallayer has been proposed. HARQ not only re-transmits data that is notreceived by a transmitter but also store data that is not received by areceiver. When the receiver receives re-transmitted data, the receiveddata is added to the previously stored data to improve a performancegain.

Since the receiver uses additional feedback radio resources to feed backan ACK/NACK signal to the transmitter in HARQ, efficient use of limitedfeedback radio resources is very important.

Hereinafter, a downlink represents a communication link from a basestation to a user equipment (UE) and an uplink represents acommunication link from the UE to the base station. The downlink is alsoreferred to as a forward link and the uplink is also referred to as areverse link. A transmitter may be a part of the base station and areceiver may be a part of the UE in the downlink. The transmitter may bea part of the UE and the receiver may be a part of the base station inthe uplink.

A method of discriminating radio resources used for downlinktransmission from radio resources such as frequency, time and codedomains used for uplink transmission is required. This method isreferred to as duplex. The uplink and the downlink can be discriminatedfrom each other in the frequency, time and code domains as does in amultiple access scheme for identifying different users. The duplex isclassified into frequency division duplexing (FDD) which discriminatesthe uplink and the downlink from each other according to frequency andtime division duplexing (TDD) which discriminates the uplink and thedownlink from each other according to time.

The uplink is discriminated from the downlink in the frequency domain inFDD, and thus transmission of data between a base station and a UE canbe continuously performed in the time domain in each link. While FDD issuitable for symmetric service such as voice communication because itsymmetrically allocates frequencies having the same level to the uplinkand the downlink, TDD is suitable for asymmetric service such asInternet service and thus researches on the TDD have been activelycarried out recently.

TDD is suitable for the asymmetric service because it can allocate timehaving different lengths to the uplink and the downlink. Furthermore,uplink data and downlink data are transmitted and received in the samefrequency band in TDD, and thus uplink and downlink channel statescorrespond to each other. Accordingly, TDD is suitable for array antennatechnology because a channel state can be immediately estimated when asignal is received. TDD uses the entire frequency band as the uplink orthe downlink, discriminates the uplink from the downlink in the timedomain, uses the frequency band as the uplink for a predetermined timeand uses the frequency band as the downlink for a predetermined time,and thus transmission and reception of data between a base station and aUE cannot be simultaneously performed.

When a base station transmits downlink data in a mobile communicationsystem, a UE transmits ACK/NACK signals with respect to the downlinkdata to the uplink after a predetermined lapse of time. If the time usedfor downlink transmission is longer than the time used for uplinktransmission, the number of ACK/NACK signals to be transmitted to theuplink may be restricted. That is, a single UE should transmit ACK/NACKsignals by using a number of ACK/NACK resources smaller than N forreceived N downlink packets. Accordingly, there is a need for anACK/NACK signal transmitting method capable of minimizing packet lossand maximizing recovery capability even when a number of ACK/NACKsignals smaller than the number of downlink packets are fed back.

SUMMARY

The present invention provides a method of effectively transmitting anACK/NACK signal in wireless communication system.

According to an aspect of the present invention, a method of performingHARQ performed by a user equipment (UE) is provided. The method includesreceiving a bundling indicator which indicates the number of bundleddownlink subframes, each subframe of the bundled downlink subframesbeing used to transmit one or plural codewords, determining whether atleast one bundled downlink subframe is missed by comparing the bundlingindicator with the number of detected bundled downlink subframes,generating a representative ACK/NACK signal when no bundled downlinksubframe is missed, wherein the representative ACK/NACK signal is an ACKsignal if all codewords in the detected bundled downlink subframes aresuccessfully received, otherwise the representative ACK/NACK signal is aNACK signal, and transmitting the representative ACK/NACK signal on anuplink channel.

The representative ACK/NACK signal may not be transmitted if at leastone bundled downlink subframe is missed.

The uplink channel may be Physical Uplink Control Channel (PUCCH). Theuplink resource for the uplink channel carrying the representativeACK/NACK signal may be associated with the last detected bundleddownlink subframe and/or the downlink resource used for the schedulingof the last detected bundled downlink subframe.

The bundling indicator may be transmitted on a downlink channel, theresource for the uplink channel is associated with the downlink channelfor the nearest bundled downlink subframe to the uplink subframe.

The uplink channel may be Physical Uplink Shared Channel (PUSCH). Thebundling indicator may be an accumulative number of the bundled downlinksubframes. The bundling indicator may be included in downlink schedulinginformation.

The bundling indicator may be included in uplink scheduling information.

According to another aspect of the present invention, a method oftransmitting a control signal performed by a UE is provided. The methodincludes receiving a bundling indicator which indicates the number ofbundled downlink subframes within M (M>1) downlink subframes,incrementing a counter when a bundled downlink subframe is detected,generating a representative NACK signal if the bundling indicator is notequal to the counter, wherein the representative NACK signal representsunsuccessful reception for all codewords in the bundled downlinksubframes, and transmitting the representative NACK signal on an uplinkshared channel in an uplink subframe.

The bundling indicator may be received on a downlink control channel.The bundling indicator may be received on a downlink control channel.The bundling indicator may be included in the bundled downlinksubframes.

The positions and the number of the bundled downlink subframes may bepredetermined with respect to the uplink subframe.

The number of the bundled downlink subframes may be equal to or biggerthan the number of the uplink subframe.

According to yet another aspect of the present invention, a method oftransmitting a control signal performed by a UE is provided. The methodincludes receiving codewords in bundled downlink subframes, incrementinga counter when a bundled downlink subframe is detected, generating arepresentative ACK/NACK signal, wherein the representative ACK/NACKsignal is a NACK signal when at least one bundled downlink subframe ismissed or when at least one codeword is not successfully received,otherwise the representative ACK/NACK signal is an ACK signal when allof the bundled downlink subframes are detected and all of the codewordsare successfully received, and transmitting the representative ACK/NACKsignal and the counter on an uplink shared channel in an uplinksubframe.

According to yet another aspect of the present invention, a method ofperforming HARQ performed by a UE is provided. The method includesreceiving bundled downlink subframes, each subframe of the bundleddownlink subframes being used to transmit one or plural codewords,generating a representative ACK/NACK signal wherein the representativeACK/NACK signal is an ACK signal if all codewords in detected bundleddownlink subframes are successfully received, and the representativeACK/NACK signal is a NACK signal if at least one codeword in thedetected bundled downlink subframes is not successfully received, andtransmitting the representative ACK/NACK signal on an uplink channel,wherein an uplink resource for the uplink channel carrying therepresentative ACK/NACK signal is associated with the last detectedbundled downlink subframe and/or the downlink resource used for thescheduling of the last detected bundled downlink subframe.

According to yet another aspect of the present invention, an apparatusfor transmitting an ACK/NACK signal using HARQ is provided. Theapparatus includes a receiving unit for receiving a bundling indicatorwhich indicates the number of bundled downlink subframes, each subframeof the bundled downlink subframes being used to transmit one or pluralcodewords, a determining unit for determining whether at least onebundled downlink subframe is missed by comparing the bundling indicatorwith the number of detected bundled downlink subframes, a generatingunit for generating a representative ACK/NACK signal when no bundleddownlink subframe is missed, wherein the representative ACK/NACK signalis an ACK signal if all codewords in the detected bundled downlinksubframes are successfully received, otherwise the representativeACK/NACK signal is a NACK signal, and a transmitting unit fortransmitting the representative ACK/NACK signal on an uplink channel.

Recovery capability is maximized and the packet loss is reduced in sucha situation that less number of ACK/NACK signals are fed back than thatof downlink packets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 is a block diagram of a UE

FIG. 3 illustrates an example of a structure of a radio frame.

FIG. 4 illustrates another example of a structure of a radio frame, thatis, TDD radio frame.

FIG. 5 illustrates the structure of the downlink subframe.

FIG. 6 illustrates the structure of the uplink subframe.

FIG. 7 illustrates transmission of an ACK/NACK signal in a PUCCH.

FIG. 8 illustrates a method of performing HARQ through ACK/NACK bundlingaccording to an embodiment of the present invention.

FIG. 9 is a flow diagram showing a method of performing HARQ in TDDsystem according to an embodiment of the present invention.

FIG. 10 illustrates a method of mapping a representative ACK/NACK signalto a radio resource according to an embodiment of the present invention.

FIG. 11 illustrates a method of transmitting an ACK/NACK signal in a TDDsystem according to an embodiment of the present invention.

FIG. 12 is a flowchart illustrating a method of transmitting an ACK/NACKsignal in a TDD system according to another embodiment of the presentinvention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention will now be described more fully with reference tothe accompanying drawings, in which exemplary embodiments of theinvention are shown. The invention may, however, be embodied in manydifferent forms and should not be construed as being limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the concept of the invention to those skilled in the art. In thedrawings, the thickness of layers and regions are exaggerated forclarity. Like reference numerals in the drawings denote like elements.

FIG. 1 shows a wireless communication system. The wireless communicationsystem can be widely deployed to provide a variety of communicationservices, such as voices, packet data, etc.

Referring to FIG. 1, the wireless communication system includes a basestation (BS) 10 and at least one user equipment (UE) 20. The BS 10 isgenerally a fixed station that communicates with the UE 20 and may bereferred to as another terminology, such as a node-B, a base transceiversystem (BTS), an access point, etc. There are one or more cells withinthe coverage of the BS 10. The UE 20 may be fixed or mobile, and may bereferred to as another terminology, such as a mobile station (MS), auser terminal (UT), a subscriber station (SS), a wireless device, etc.

A downlink represents a communication link from the BS 10 to the UE 20,and an uplink represents a communication link from the UE 20 to the BS10. In downlink, a transmitter may be a part of the BS 10, and areceiver may be a part of the UE 20. In uplink, the transmitter may be apart of the UE 20, and the receiver may be a part of the BS 10.

Different multiple access schemes may be used for downlink and uplinktransmissions. For example, orthogonal frequency division multipleaccess (OFDMA) is used for downlink, and single carrier-frequencydivision multiple access (SC-FDMA) is used for uplink.

There is no limit in the multiple access scheme used in the wirelesscommunication system. The multiple access scheme may be based on codedivision multiple access (CDMA), time division multiple access (TDMA),frequency division multiple access (FDMA), SC-FDMA, OFDMA, or otherwell-known modulation schemes. In these modulation schemes, signalsreceived from multi-users are demodulated to increase capacity of thecommunication system.

The layers of the radio interface protocol can be classified into afirst layer (L1), a second layer (L2), and a third layer (L3) based onthe open system interconnection (OSI) model that is well-known in acommunication system. Compared with the OSI model, a physical (PHY)layer corresponds to the first layer L1, the medium access control (MAC)layer and the radio link control (RLC) layer correspond to the secondlayer L2, and the radio resource control (RRC) layer corresponds to thethird layer L3. The physical layer provides an information transferservice using a physical channel, the MAC layer is connected with thephysical layer through transport channel, and the radio RRC layer servesto control radio resources between the UE and the network.

Examples of a downlink transport channel include a broadcast channel(BCH) for transmitting system information and a downlink-shared channel(DL-SCH) for transmitting user traffic or control messages. The usertraffic of downlink multicast or broadcast services or the controlmessages can be transmitted on the DL-SCH or an additional downlinkmulticast channel (DL-MCH). The downlink transport channel is mapped toa downlink physical channel.

Examples of the downlink physical channel include a physical downlinkshared channel (PDSCH) mapped to the DL-SCH, and a physical downlinkcontrol channel (PDCCH) for transmitting a control signal.

Examples of an uplink transport channel include a random access channel(RACH) for transmitting initial control messages and an uplink-sharedchannel (UL-SCH) for transmitting user traffic or control messages. Theuplink transport channel is mapped to a physical uplink channel.Examples of the physical uplink channel include a physical random accesschannel (PRACH) mapped to the RACH, a physical uplink shared channel(PUSCH) mapped to the UL-SCH, and a physical uplink control channel(PUCCH) for uplink control messages. The PUSCH is a physical uplinkshared channel, and is used when the UE transmits data in uplink.

The PDCCH is a downlink physical control channel and transmits controlinformation for the PUSCH or the PDSCH. An uplink grant that isscheduling information for uplink data transmission and a downlink grantthat is scheduling information for downlink data transmission can betransmitted through the PDCCH. Herein, the scheduling informationimplies control information including radio resource allocation fortransmitting downlink data from the BS to the UE or for receiving uplinkdata from the UE, a modulation and coding scheme (MCS), MIMOinformation, etc.

FIG. 2 is a block diagram of a UE 50. The UE 50 includes a processor 51,a memory 52, an RF unit 53, a display unit 54, and a user interface unit54. The processor 51 implements layers of air interface protocol andprovides a control plane and a user plane. Functions of the layers areimplemented through the processor 51. The memory 52 is connected to theprocessor 51 and stores a UE driving system, application and generalfiles. The display unit 54 displays information on the UE and may usewell-known components such as organic light emitting diodes (OLEDs). Theuser interface unit 55 may be composed of a combination of well-knownuser interfaces such as a keypad or a touch screen. The RF unit 53 isconnected to the processor and transmits and/or receives radio signals.

FIG. 3 illustrates an example of a structure of a radio frame.

Referring to FIG. 3, the radio frame has ten subframes and each subframemay include two slots. A basic data transmission unit corresponds to asubframe and scheduling of downlink or uplink is performed based on thesubframe. A single slot may include a plurality of OFDM symbols in thetime domain and at least one subcarrier in the frequency domain. Thesingle slot may include six or seven OFDM symbols.

FIG. 4 illustrates another example of a structure of a radio frame, thatis, TDD radio frame.

Referring to FIG. 4, the radio frame includes two half-frames. Thehalf-frames have the same structure. Specifically, each half-frameincludes 5 subframes and 3 fields, i.e., a downlink pilot time slot(DwPTS), a guard period (GP), and an uplink pilot time slot (UpPTS). TheDwPTS is used for initial cell search, synchronization, or channelestimation in a UE. The UpPTS is used for channel estimation in a BS anduplink transmission synchronization of the UE. The GP is used to removeinterference that occurs in uplink due to a multi-path delay of adownlink signal between uplink and downlink.

Table 1 shows an example of a configuration of the radio resource. Theconfiguration of the radio frame indicates a specific rule according towhich all subframes are allocated (or reserved) for uplink or downlink.

TABLE 1 Config- Switch-point Subframe number uration periodicity 0 1 2 34 5 6 7 8 9 0 5 ms D S U U U D S U U U 1 5 ms D S U U D D S U U D 2 5 msD S U D D D S U D D 3 10 ms  D S U U U D D D D D 4 10 ms  D S U U D D DD D D 5 10 ms  D S U D D D D D D D 6 5 ms D S U U U D S U U D

In Table 1, ‘D’ denotes a subframe used for downlink transmission, and‘U’ denotes a subframe used for uplink transmission. Further, ‘S’denotes a special subframe that is used for a special purpose, such as,frame synchronization or downlink transmission. Hereinafter, a subframeused for downlink transmission is simply referred to as a downlinksubframe, and a subframe used for uplink transmission is simply referredto as an uplink subframe. For each configuration, a position and thenumber of downlink and uplink subframes are different from each otherwithin one radio frame.

A time point at which downlink is switched to uplink, or vice versa, isdefined as a switching point. A switch-point periodicity represents aperiod in which the same switching pattern is repeated between uplinkand downlink. The switch-pint periodicity is 5 ms or 10 ms. For example,in case of the configuration 1, switching occurs in the pattern ofD→S→U→U→U from 0th to 4th subframes. In addition, from 5th to 9thsubframes, switching occurs in the pattern of D→S→U→U→U in the samepattern as the previous switching. Since one subframe is 1 ms, theswitch-point periodicity is 5 ms. That is, the switch-point periodicityis less than one radio frame length (i.e., 10 ms), and switching isrepeated one time within the radio frame. For all configurations, 0thand 5th subframes and the DwPTS are used for downlink transmission. A1st subframe in all configurations and a 6th subframe in configurations0, 1, 2, and 6 are composed of the DwPTS, the GP, and the UpPTS. A timelength of each field varies depending on configurations. The remaining 8subframes other than the 1st and 6th subframes are composed of 2 slots.

If the switch-point periodicity is 5 ms, the UpPTS and 2nd and 7thsubframes are reserved for uplink transmission. If the switch-pointperiodicity is 10 ms, the UpPTS and the 2nd subframe are reserved foruplink transmission, and the DwPTS, and 7th and 9th subframes arereserved for downlink transmission.

The configuration of Table 1 may be system information known by both theBS and the UE. The BS can inform the UE that an uplink-downlinkallocation state of the radio frame is modified by transmitting only aconfiguration index whenever the configuration of the radio framechanges. The configuration is a sort of downlink control information.Similarly to other scheduling information, the configuration may betransmitted on a physical downlink control channel (PDCCH).Alternatively, the configuration may be control information commonlytransmitted on a broadcast channel to all UEs existing in a cell. Inaddition, the configuration may be included in the system information.The number of half-frames included in the radio frame, the number ofsubframe included in the half-frame, and the combination of the downlinksubframe and the uplink subframe in the TDD system are shown forexemplary purposes only.

In Table 1, if ‘S’ corresponds to downlink subframes, a single radioframe includes 8 downlink subframes and 2 uplink subframes in the caseof configuration 2. That is, the ratio of the number of downlinksubframes to the number of uplink subframes is 4:1. In this case, the UEmust receive data through 4 downlink subframes and transmit ACK/NACKsignals through a single uplink subframe. When the number of downlinksubframes is greater than the number of uplink subframes as describedabove, uplink radio resources are insufficient for one-to-one mappingbetween subframe(s) for downlink data and subframe(s) for ACK/NACKsignals. Accordingly, N:1 mapping is carried out between subframe(s) fordata and subframe(s) for ACK/NACK signals. In this case, A singleACK/NACK signal is used as an HARQ feedback for transmission of pluralPDSCHs for a single UE. This is referred to as ACK/NACK bundling.

FIG. 5 illustrates the structure of the downlink subframe.

Referring to FIG. 5, 3 prior OFDM symbols of the first slot of thedownlink subframe correspond to a control region to which a PDCCH isallocated and remaining OFDM symbols correspond to a data region towhich a PDSCH is allocated. Control channels other than the PDCCH, suchas PCFICH and PHICH, may be allocated to the control region. A UE maydecode control information transmitted through the PDCCH to read datainformation transmitted through the PDSCH. Here, the number of OFDMsymbols included in the control region in the subframe is not limited to3 and can be known through PCFICH.

The control region is composed of a set of control channel elements(CCEs). This CCE set is a set of CCEs constructing the control region inthe single subframe. The CCE corresponds to a plurality of resourceelement groups. For example, the CCE may correspond to 9 resourceelement groups. The resource element group is used to define mapping ofcontrol channels to resource elements. For instance, a single resourceelement group can be composed of 4 resource elements.

A plurality of multiplexed PDCCHs for a plurality of UEs may betransmitted within the control region. The PDCCH transfers controlinformation such as scheduling allocation. The PDCCH is transmitted onaggregation of a single CCE or several contiguous CCEs. Hereinafter, thenumber of CCEs used for PDCCH transmission is referred to as a CCEaggregation level. For example, the CCE aggregation level may beelements of {1, 2, 4, 8}. The CCE aggregation level corresponds to thenumber of CCEs used for PDCCH transmission and is a CCE unit forsearching PDCCH. The magnitude of the CCE aggregation level is definedby the number of contiguous CCEs. The CCE aggregation level may differdepending on UE. In FIG. 4, for instance, the CCE aggregation level ofthe 2nd, 4th and 6th UEs UE2, UE4 and UE6 is 1, the CCE aggregationlevel of the 3rd and 5th UEs UE3 and UE5 is 2, and the CCE aggregationlevel of the 1st and 7th UEs UE1 and UE7 is 4.

FIG. 6 illustrates the structure of the uplink subframe.

Referring to FIG. 6, the uplink subframe can be divided into a controlregion to which a PDCCH that transfers uplink control information in thefrequency domain is allocated and a data region to which a PUSCH thattransfers user data is allocated.

The PUCCH for the single UE is assigned to a resource block (RB) pair inthe subframe and RBs belonging to the RB pair respectively havedifferent subcarriers in two slots. That is, the RB pair allocated tothe PUCCH frequency-hops at the slot boundary.

The PUCCH can support multiple formats. That is, the PUCCH can transmituplink control information having different numbers of bits forsubframes according to modulation scheme. The following table representsPUCCH format, modulation scheme and the number of bits, which aresupported according to 3GPP TS 36.211 V8.2.0.

TABLE 2 PUCCH format Modulation scheme Number of bits per subframe, M 1 N/A N/A 1a BPSK 1 1b QPSK 2 2  QPSK 20 2a QPSK + BPSK 21 2b QPSK + QPSK22

PUCCH format 1 is used to transmit scheduling request SR, PUCCH format1a/1b is used to transmit a representative ACK/NACK signal, PUCCH format2 is used to transmit CQI, and PUCCH format 2a/2b is used to transmitCQI and the representative ACK/NACK signal.

The PUCCH format 1a/1b is used when a representative ACK/NACK signal istransmitted alone in an arbitrary subframe and the PUCCH format 1 isused when SR is transmitted alone. PUCCH formats for transmittingcontrol information have been described above. An allocation scheme andallocation quantity of radio resources used to transmit the controlinformation may differ depending on the PUCCH format.

FIG. 7 illustrates transmission of an ACK/NACK signal in a PUCCH.

Referring to FIG. 7, a reference signal RS is loaded in 3 SC-FDMAsymbols among 7 SC-FDMA symbols included in a single slot and arepresentative ACK/NACK signal is loaded in the remaining four SC-FDMAsymbols. The reference signal SC is loaded in 3 contiguous SC-FDMAsymbols in the middle of the slot.

To transmit the ACK/NACK signal, a 2-bit representative ACK/NACK signalis QPSK-modulated to generate a single modulation symbol d(0). Amodulated sequence m(n) is generated based on the modulation symbol d(0)and the cyclically shifted sequence r(n,a). ‘a’ is an amount of cyclicshift (CS). It is possible to multiply the cyclically shifted sequencer(n,a) by the modulation symbol to generate a modulated sequence y(n)according to Equation 1.

y(n)=d(0)r(n,a)   [Equation 1]

The cyclically shifted sequence r(n,a) may have different CS amounts orthe same CS amount for the respective SC-FDMA symbols. Although CSamounts 0, 1, 2 and 3 are sequentially set for 4 SC-FDMA symbols in asingle slot in this case, this is exemplary.

Furthermore, although the single modulation symbol is generated byQPSK-modulating the 2-bit ACK/NACK signal in the current embodiment, itis also possible to BPSK-modulate a 1-bit ACK/NACK signal to generate asingle modulation symbol. The number of bits of the ACK/NACK signal,modulation scheme and the number of modulation symbols are exemplary anddo not limit the technical spirit of the present invention.

Furthermore, the modulated sequence can be spread using an orthogonalsequence in order to increase the capacity of a UE. Sequencesrepresented in Table 3 can be used as orthogonal sequences w_(i)(k) (iis a sequence index, 0≦k≦K−1) having a spreading factor of K=4.

TABLE 3 Sequence index [w(0), w(1), w(2), w(3)] 0 [+1 +1 +1 +1] 1 [+1 −1+1 −1] 2 [+1 −1 −1 +1]

Otherwise, sequences represented in Table 4 can be used as orthogonalsequences w_(i)(k) (i is a sequence index, 0≦k≦K−1) having a spreadingfactor of K=3.

TABLE 4 Sequence index [w(0), w(1), w(2)] 0 [1 1 1] 1 [1 e^(j2π/3)e^(j4π/3)] 2 [1 e^(j4π/3) e^(j2π/3)]

Here, an operation of spreading a sequence modulated through theorthogonal sequence w_(i)(k) having a spreading factor of K=4 for 4SC-FDMA symbols included in a single slot for a representative ACK/NACKsignal is described.

FIG. 8 illustrates a method of performing HARQ through ACK/NACK bundlingaccording to an embodiment of the present invention. FIG. 8 shows that aUE which has received downlink data transmits ACK/NACK signals to uplinkin a TDD system in which the number of subframes used for downlinktransmission is greater than the number of subframes used for uplinktransmission. But the present invention is applied not only to a TDDsystem but also to a FDD system where ACK/NACK signals for pluraldownlink subframes are transmitted via a single uplink subframe

Referring to FIG. 8, the UE transmits data corresponding to 3 contiguousdownlink subframes or a single ACK/NACK signal corresponding to a PDSCHthrough a single uplink subframe. That is, the radio of the number ofACK/NACK signals to the data (or PDSCH) is 3:1. When a plurality ofsubframes are allocated to a specific UE for downlink transmission inthe TDD system, ACK/NACK signals are transmitted in consideration ofdata transmitted to the downlink as a single HARQ packet.

Hereinafter, an ACK/NACK signal transmitted through a single uplinksubframe according to ACK/NACK bundling is referred to as arepresentative ACK/NACK signal. And among a plurality of downlinksubframes associated with the single uplink subframe, at least onedownlink subframe by which data for a certain UE is transmitted isreferred to as bundled downlink subframes. And each of the subframeincluded in the bundled downlink subframes is referred to as a bundleddownlink subframe.

The UE determines the representative ACK/NACK signal as an ACK or NACKsignal according to the following method. The UE performs decoding for acodeword that the US received in a bundled downlink subframe, andexecutes a logic AND operation on ACK signals or NACK signals forbundled downlink subframes to generate at least one representativeACK/NACK signal. That is, the UE transmits an ACK signal only whensuccessively receiving all the codewords received on the bundleddownlink subframes and transmits a NACK signal when failing in receivingany one of the codewords. Or, when a plurality of representativeACK/NACK signals is generated, the UE divides codewords, which istransmitted over bundled downlink subframes, into a plurality ofcodeword groups. So the UE can transmit ACK/NACK signal for each of thecodeword group.

This is a principal of determining an ACK or NACK signal inconsideration of downlink data composed of a plurality of codewords assingle data. Here, a codeword is a unit of data transmitted for everybundled downlink subframe and may be referred to as a transport block.

Hereinafter, a set of codewords corresponding to the basis of decisionin generating a representative ACK/NACK signal is referred to as adownlink data packet. Accordingly, it is considered that the UEsuccessively receives the downlink data packet when the representativeACK/NACK signal is an ACK signal and it is considered that the UE failsin receiving the downlink data packet when the representative ACK/NACKsignal is a NACK signal.

If a UE fails to decode downlink data packet, it is natural for the UEto transmit a NACK signal to a BS. And the BS retransmits the downlinkdata packet to the UE. But, if the UE misses scheduling information(orPDCCH) and cannot detect the existence of data of the missed bundleddownlink subframe, the UE should perform HARQ only based on the rest ofbundled downlink subframe(s) apart from the missed bundled downlinksubframe. It doesn't matter when the UE is supposed to transmit a NACKsignal, because the BS can retransmit downlink data. But when the UE issupposed to transmit an ACK signal, the data for the missed bundleddownlink subframe is lost and cannot be recovered. Therefore, the BS orthe UE need to know to which bundled downlink subframe therepresentative ACK/NACK signal is corresponds.

In FIG. 8, although it is assumed that the UE is the subject thattransmits the ACK/NACK signal, the BS can also transmit the ACK/NACKsignal in the same manner.

FIG. 9 is a flow diagram showing a method of performing HARQ in TDDsystem according to an embodiment of the present invention.

Referring to FIG. 9, a BS transmits a bundling indicator to a UE in stepS100. The bundling indicator is control information which indicatesbundled downlink subframes associated with a single uplink subframe fortransmission of a representative ACK/NACK signal. The bundling indicatormay be the number of bundled downlink subframe(s), or a transmissionorder of bundled downlink subframe(s). The bundling indicator isincluded in scheduling information and can be transmitted through aPDCCH. Otherwise, the bundling indicator can be included in bundleddownlink data packets and transmitted. The bundling indicator will bedescribed in more detail later.

The BS transmits to a specific UE a downlink data packet over bundleddownlink subframes in step S110. The bundled downlink subframes may becontiguous subframes or discontinuous subframes. The downlink datapacket is transmitted through a PDSCH that is a physical channel inevery downlink subframe. When the bundling indicator is included in thebundled downlink data packet, the bundling indicator and the bundleddownlink data packet can be simultaneously transmitted.

The UE detects the bundled downlink subframe(s) in step S120. The UEdetermines that there is no missing of bundled downlink subframe whenall the bundled downlink subframes indicated by the bundling indicatorare successfully received. And the UE determines that there is a missingbundled downlink subframe when failing in receiving any bundled downlinksubframe. For instance, if the UE detects only two downlink subframesalthough the bundling indicator indicates that there are three bundleddownlink subframes. The UE determines that a bundled downlink subframeis missed and generates a representative NACK signal to report that theUE failed in detecting one bundled downlink subframe. Of course, the UEdetermines that there is a reception error when it fails in receivingany one of the two downlink subframes.

The UE transmits the representative ACK/NACK signal to the BS in stepS130. The UE transmits the representative ACK/NACK signal through apredetermined uplink subframe. The representative ACK/NACK signal may betransmitted through a PUCCH that is an uplink control channel or a PUSCHthat is an uplink data channel. The uplink radio resources used totransmit the representative ACK/NACK signal will be described later inmore detail.

The BS re-transmits HARQ when the representative ACK/NACK signal is aNACK signal and transmits new data when the representative ACK/NACKsignal is an ACK signal in step S140.

The bundling indicator will now be explained in detail. In anembodiment, the bundling indicator may correspond to the number ofbundled downlink subframes. The bundling indicator is 3 in FIG. 8, forexample. The bundling indicator may be transmitted through all thebundled downlink subframes or some of the bundled downlink subframes.The UE counts the number of bundled downlink subframes that the UEdetects. Then, the UE compares the bundling indicator with a counter. Ifthe bundling indicator is different from the counter of the UE, the UEtransmits a representative NACK signal to the BS or does not perform anyoperation and operates in a discontinuous transmission (DTX) mode. Ifthe number of downlink subframes actually counted by the UE is 2 while abundling indicator 3 is received, for instance, it means that UE failsin receiving a single downlink subframe, and thus the UE transmits therepresentative NACK signal to the BS or does not perform any operation.

In another embodiment, the bundling indicator can indicate atransmission order of bundled downlink subframes or a transmission orderof PDSCH with respect to a specific UE. The UE counts the number ofdownlink subframes detected by itself. For instance, if 4 downlinksubframes are associated with a single uplink subframe for arepresentative ACK/NACK signal, and 3 out of the 4 downlink subframesare bundled downlink subframes for a specific UE. Bundling indicators 1,2 and 3 can be accumulatively assigned to the three bundled downlinksubframes in sequence. The specific UE increases the counter wheneverthe UE detects a bundled downlink subframe. If the specific UE detectsonly bundled downlink subframes corresponding to the bundling indicators1 and 3, for instance. The counter is 1 when the specific UE detects the1st bundled downlink subframe. The specific UE fails in detecting the2nd bundled downlink subframe, and thus the counter is still 1. When thespecific UE detects the 3rd bundled downlink subframe, the counter is 2although the bundling indicator is 3. The bundling indicator does notcorrespond to the counter, and thus it can be known that the specific UEfails in detecting PDCCH of the 2nd bundled downlink subframecorresponding to the bundling indicator 1. In this case, the specific UEmay generate a representative NACK signal or do not perform anyoperation and operate in the DTX mode.

An example that the UE determines that there is a reception error isrepresented as follows.

Bundling indicator≠(N_(DAI)−1)mod(a)+1   [Equation 2]

Here, N_(DAI) denotes the number of bundled downlink subframessuccessfully detected by the UE and mod(a) represents a modulooperation. When the bundling indicator is ceiling[log₂(a)] bitinformation, the transmission order starts from 1 when the bundlingindicator becomes greater than a maximum transmission order a, and thusthe modulo operation is performed. In this manner, the UE can correctlytransmit a representative ACK/NACK signal. Here, the bundling indicatorcan be also referred to as a downlink assignment index (DAI).

The information indicated by the bundling indicator may differ dependingon DCI format. For example, when the DCI format is 0, the bundlingindicator simply indicates the number of PDSCH transmissions associatedwith an uplink subframe which transmits a representative ACK/NACK signalfor a specific UE. When the DCI format is 1/1A/1B/1D/2/2A, which is fordownlink scheduling information, the bundling indicator indicates theaccumulative number of PDSCH transmissions for the specific UE, and canbe updated every subframe.

Radio resources used for a UE to transmit a representative ACK/NACKsignal will now be explained. In ACK/NACK bundling, a single uplinksubframe is used to transmit ACK/NACK signals for bundled downlinksubframes. Accordingly, radio resources that will transmit the ACK/NACKsignals with respect to the plurality of bundled downlink subframes mustbe allocated to the uplink subframe. If downlink data is transmitted todifferent UEs through 4 downlink subframes when the ratio of the numberof downlink subframes to the number of uplink subframes is 4:1, forexample, the UEs should be able to transmit the ACK/NACK signals throughthe same uplink subframe. Accordingly, a radio resource for ACK/NACKsignals, which is four times the radio resource required when the ratioof the number of downlink subframes to the number of uplink subframes is1:1, must be allocated to each uplink subframe.

On the contrary, when downlink data is transmitted to a single UEthrough bundled downlink subframes, a radio resource for arepresentative ACK/NACK signal corresponding to the bundled downlinksubframes will not be used by other UEs. Accordingly, the single UE cantransmit the representative ACK/NACK signal by using one of radioresources allocated for ACK/NACK signals.

According to the present invention, a UE transmits a representativeACK/NACK signal using a radio resource for ACK/NACK signals and theradio resource for the ACK/NACK signals are assigned for the lastbundled downlink subframe in the bundled downlink subframes. This givesa criterion in which a BS determines whether PDSCH of the bundleddownlink subframes is successfully transmitted or not. For example, if aUE misses the last bundled downlink subframe(s) in the bundled downlinksubframes, the UE shall transmit a representative ACK/NACK signal usinga radio resource assigned for the second last bundled downlink subframe.In this case, the BS can find out the last bundled downlink subframe(s)is missed, by the radio resource used to transmit the representativeACK/NACK signal.

FIG. 10 illustrates a method of mapping a representative ACK/NACK signalto a radio resource according to an embodiment of the present invention.Here, the representative ACK/NACK signal is transmitted through a PUCCH.

Referring to FIG. 10, a single uplink subframe is allocated to transmitan ACK/NACK signal for 4 downlink subframes. The specific position ofthe bundled downlink subframe(s) corresponding to an uplink subframe forthe representative ACK/NACK signal, is defined a table below.

TABLE 5 Config- Subframe n uration 0 1 2 3 4 5 6 7 8 9 0 — — 6 — 5 — — 6— 4 1 — — 7, 6 4 — — — 7, 6 4 — 2 — — 8, 7, — — — — 8, 7, — — 6, 4 6, 43 — — 11, 7, 6 6, 5 5, 4 — — — — — 4 — — 12, 11, 7, 6, — — — — — — 8, 75, 4 5 — — TBD — — — — — — — 6 — — 7 7 5 — — 7 7 —

In Table 5, the subframe n denotes an uplink subframe index. A bundleddownlink subframe corresponding to an nth subframe (uplink subframe) isdetermined as n−k. That is, the bundled downlink subframe is indicatedas a subframe prior to the nth subframe by k. Here, k denotes anindicator which determines the bundled downlink subframe and belongs toa set K including M elements {k₀, k₁, . . . , k_(M−1)}. The position ofthe bundled downlink subframe is determined by the subframe n andconfiguration.

For example, if the representative ACK/NACK signal is transmittedthrough the 2nd subframe (n=2) when a radio frame is determined byconfiguration 2, the bundling indicator when n=2 corresponds toK={8,7,6,4}. Accordingly, bundled downlink subframes covered by therepresentative ACK/NACK signal correspond to 4th, 5th, 6th and 8thsubframes prior to the 2nd subframe by 8, 7, 6 and 4. This is becausethe bundled downlink subframes are prior to the uplink subframe whichtransmits the representative ACK/NACK signal.

Each downlink subframe is composed of a PDCCH and a PDSCH and the PDCCHis segmented into 4 sections. The section can be referred to as acertain region of radio resources (i.e. resource block (RB)). Thissegmentation is exemplary and the interval and number of segmentedsections can be varied. A UE decodes the PDCCH through CCE, and thendecodes the PDSCH according to downlink grant of the PDCCH. The UEreceives data from the PDSCH and a region receiving the data may becomposed of resource blocks which are radio resource units.

Radio resources for ACK/NACK signals for a specific PDCCH can bedetermined by a resource index. Here, the radio resources mean radioresources of the PUCCH. For example, the 0th downlink subframe usesradio resources corresponding to indexes 1, 2, 3 and 4 in the uplinksubframe to transmit ACK/NACK signals and the 1st downlink subframe usesradio resources corresponding to indexes 5, 6, 7 and 8 in the uplinksubframe to transmit the ACK/NACK signals. That is, radio resources usedto transmit ACK/NACK signals through a single uplink subframe can beclassified for the respective downlink subframes. Or to reduce theamount of radio resources for ACK/NACK signal transmission, it can beconsidered that the ACK/NACK signal resource for a certain downlinksubframe can be duplicated with other downlink subframe.

If a specific UE receives a codeword through 0th, 1st and 3rd downlinksubframes among the 4 bundled downlink subframes, the specific UEdecodes section 3 of PDCCH of the 0th downlink subframe and reads PDSCHindicated by section 3. Further, the specific UE decodes section 6 ofPDCCH of the 1st downlink subframe and reads PDSCH indicated by section6. In addition, the specific UE decodes section 13 of PDCCH of the 3rddownlink subframe and reads PDSCH indicated by section 13.

The specific UE transmits a representative ACK/NACK signal according towhether the UE successfully receives the PDSCH or fails in receiving thePDSCH. Here, the specific UE does not use radio resource indexes 3 and 6and transmits the representative ACK/NACK signal by using only radioresource index 13 corresponding to the last section 13 which has beenreceived latest by the UE.

A radio resource of a bundled downlink subframe(s) that a UE detects themost recently(or nearest to the uplink subframe n), is used to transmitthe representative ACK/NACK signal. The bundled downlink subframe(s) isincluded in downlink subframe associated with the uplink subframe n.

For example, it is assumed that the representative ACK/NACK signal istransmitted in uplink subframe 2 in configuration 2. Bundled downlinksubframes are 4th, 5th, 6th and 8th subframes. If the UE detects the4th, 5th and 8th subframes, the representative ACK/NACK signal istransmitted using the radio resource according to the last received 8thdownlink subframe. This is performed when k=4. That is, when a downlinksubframe according to the smallest number of elements which belong tothe set K is determined, the radio resource used to transmit therepresentative ACK/NACK signal is determined according to the downlinksub frame.

This is a method of determining radio resources which transmit therepresentative ACK/NACK signal. A method of determining whether arepresentative ACK/NACK signal corresponds to an ACK signal or a NACKsignal has been described above.

The specific UE cannot read the PDSCH indicated by section 13 if the UEdoes not detect section 13 of the PDCCH of the 3rd downlink subframe.Accordingly, the specific UE will transmit the representative ACK/NACKsignal according to a radio resource index corresponding to the lastreceived section 6. On the contrary, a BS expects to receive therepresentative ACK/NACK signal according to radio resource index 13because the BS transmits downlink data according to section 13. However,the BS receives the representative ACK/NACK signal according to radioresource index 6, and thus the BS can recognize that transmission ofdownlink data last transmitted according to section 13 failed.Accordingly, the BS can perform HARQ re-transmission even if therepresentative ACK/NACK signal is an ACK signal.

If the method of selecting radio resource for ACK/NACK signal iscombined with the method of detecting bundled downlink subframe withbundling indicator described above, it is possible to discover missedbundled downlink subframe(s) regardless of the position of the missedbundled downlink subframe(s) is in the middle or at the last. Forexample, it is assumed that the specific UE does not detect section 6 ofthe PDCCH of the 1st downlink subframe and detects section 13. Thebundling indicator in the PDCCH of section 3 is 1, the bundlingindicator in the PDCCH of section 6 is 2, and the bundling indicator inthe PDCCH of section 13 is 3. The UE fails in detecting section 6, andthus the counter of the UE when the UE detects the PDCCH of section 13still indicates 2. Here, the UE does not transmit the representativeACK/NACK signal according to the radio resource corresponding to section13 and operates in the DTX mode. The BS expects to receive therepresentative ACK/NACK signal according to the radio resource index 13because the BS has transmitted the downlink data according to section13. However, the BS does not receive any representative ACK/NACK signal,and thus the BS can perform HARQ re-transmission.

The BS can discern the successful or unsuccessful transmission ofbundled downlink subframes, with the radio resource for a representativeACK/NACK signal transmitted from the UE. And the UE can discern whethera bundled downlink subframe(s) is missed by comparing a bundlingindicator with the number of detected bundled downlink subframes.

A method of transmitting an ACK/NACK signal by using radio resourcesused to transmit uplink data will now be explained. When a UEsimultaneously transmits the uplink data when transmitting the ACK/NACKsignal to an uplink subframe, a radio resource allocated to transmit theACK/NACK signal may be not a radio resource for transmitting generalcontrol information. That is, the UE can transmit the ACK/NACK signal byusing a radio resource in the PUSCH without using a radio resource inthe PUCCH. Here, the ACK/NACK signal may be multiplexed with generaluplink data.

The UE transmits a NACK signal with respect to downlink data that is notrecognized or not for the UE as well as downlink data which is judged bythe UE whether it is successfully received. A NACK signal formallytransmitted for downlink data is referred to as a dummy NACK signal.When the dummy NACK signal is transmitted, a BS can be aware whether theUE properly receives the downlink data and whether the ACK/NACK signalstransmitted from the UE are true.

FIG. 11 illustrates a method of transmitting an ACK/NACK signal in a TDDsystem according to an embodiment of the present invention.

Referring to FIG. 11, there are 4 downlink subframes #0, #1, #2 and #3and an plink subframe #n for transmitting ACK/NACK signals correspondingto the 4 downlink subframes #0, #1, #2 and #3. Only the downlinksubframe #0 transmits downlink data for a UE A. Accordingly, the UEdecodes the downlink subframe #0 and transmits an ACK/NACK signal withrespect to the downlink subframe #0 by using a radio resource ‘a’ of thePUSCH of the uplink subframe #n. Further, the UE transmits 3 NACKsignals as ACK/NACK signals with respect to the downlink subframes #1,#2 and #3 by using radio resources ‘b’, ‘c’ and ‘d’ of the PUSCH of theuplink subframe #n. The UE transmits all the ACK/NACK signals withrespect to the downlink subframes #1, #2 and #3 even though downlinkdata for the UE A is not transmitted in the downlink subframes #1, #2and #3. That is, the UE transmits 4 ACK/NACK signals.

Although the radio resources a, b, c and d are continuous in FIG. 11,radio resources for transmitting ACK/NACK signals can be scattered inthe PUSCH. Furthermore, the method of mapping ACK/NACK signals to theradio resources a, b, c and d may be different from a radio resourcemapping method for ACK/NACK signals in the PUCCH. Further, an ACK/NACKsignal may be independently transmitted or multiplexed with uplink dataand transmitted.

The ACK/NACK signal may be transmitted through the PUCCH if required. Inthis case, a representative ACK/NACK signal is transmitted as describedabove. The system can freely select whether the representative ACK/NACKsignal is transmitted through the PUCCH or a dummy NACK signal istransmitted through the PDSCH. Further, an ACK/NACK signal may bedynamically transmitted while switching to the PUCCH and PUSCH.

To transmit the same amount of ACK/NACK signals with the number ofmaximum downlink subframes or data for a UE, causes an overhead problembecause the amount of feedback increases. Hereinafter, a method forfeedback reduction discloses.

FIG. 12 is a flowchart illustrating a method of transmitting an ACK/NACKsignal in a TDD system according to another embodiment of the presentinvention.

Referring to FIG. 12, a BS configures bundled downlink subframes withN_(D) downlink subframes, and transmits data to a UE over the bundleddownlink subframes in step S200. The downlink data is transmittedthrough PDSCH of the bundled downlink subframes. The UE transmits arepresentative ACK/NACK signal to the BS in step S210. The method ofgenerating the representative ACK/NACK signal has been described withreference to FIG. 8. The UE transmits data counting information N_(C) tothe BS in step S220. N_(C) Is the number of bundled downlink subframe(s)that the UE detects. So, N_(D)=N_(C)+N_(missed). Here, N_(missed)denotes the amount of bundled downlink subframes that is not detected(or recognized) by the UE.

The number of bits of the data counting information isceiling[log₂(max(N_(D)))], if the data counting information is expressedin an independent information bit stream. The UE must transmitinformation of ceiling[ log₂(max(N_(D))]]+2 bits through an uplinksubframe, if the representative ACK/NACK signal is 2 bit. Therefore, itis more desirable in signaling overhead aspect to use both therepresentative ACK/NACK signal and the data counting information, thanto transmit 2×max(N_(D))−bit ACK/NACK signals. The representativeACK/NACK signal and the data counting information are transmittedthrough a PUSCH.

The BS determines whether HARQ re-transmission is performed or new datais transmitted according to the representative ACK/NACK signal and thedata counting information in step S230. This determination is performedby comparing N_(D) and N_(C) by the BS. In an embodiment, when the UEdetects all the bundled downlink subframes, N_(missed)=0 andN_(C)=N_(D). That is, the BS can be aware that no bundled downlinksubframes are missed from the data counting information. Therefore, theBS performs HARQ retransmission or new data transmission according tothe representative ACK/NACK signal.

In another embodiment, when the UE misses at least one of the bundleddownlink subframes, N_(missed)≠0 and N_(C)≠N_(D). Therefore, the BS canbe aware that the UE missed N_(missed) bundled downlink subframe(s).Then, the BS can perform HARQ re-transmission even if the BS receives arepresentative ACK signal from the UE.

The aforementioned functions can be executed by processors such asmicroprocessors, controllers, microcontrollers, application specificintegrated circuits (ASICs) and so on according to software or programcodes coded to execute the functions. The design, development andimplementation of the codes are obvious to those skilled in the art.

While the present invention has been particularly shown an describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

What is claimed is:
 1. A method for transmitting a receptionacknowledgement in a wireless communication system, performed by a userequipment, the method comprising: receiving at least one downlink packetin a plurality of Time Division Duplex (TDD) downlink subframes from abase station; determining an ACK/NACK signal based on whether the atleast one downlink packet is missed in the plurality of TDD downlinksubframes; and transmitting an ACK/NACK signal on an uplink datachannel, wherein, if the at least one downlink packet is determined tohave been missed, the ACK/NACK signal is determined as a NACK, andwherein, if no downlink packet is determined to have been missed, theACK/NACK signal is determined by a logical AND operation of ACK/NACKsfor the at least one received downlink packet.
 2. The method of claim 1,further comprising: receiving information about a total number of theplurality of TDD downlink subframes from the base station.
 3. The methodof claim 2, wherein whether the at least one downlink packet is missedis determined based on the total number of the plurality of TDD downlinksubframes and a number of the at least one received downlink packet 4.The method of claim 2, further comprising: receiving an uplinkassignment for the uplink data channel from the base station.
 5. A userequipment configured for transmitting a reception acknowledgement in awireless communication system, the user equipment comprising: a radiofrequency unit configured to receive and transmit radio signals; and aprocessor operatively coupled with the radio frequency unit andconfigured to: receive at least one downlink packet in a plurality ofTime Division Duplex (TDD) downlink subframes from a base station;determine an ACK/NACK signal based on whether the at least one downlinkpacket is missed in the plurality of TDD downlink subframes; andinstruct the radio frequency unit to transmit an ACK/NACK signal on anuplink data channel, wherein, if the at least one downlink packet isdetermined to have been missed, the ACK/NACK signal is determined as aNACK, and wherein, if no downlink packet is determined to have beenmissed, the ACK/NACK signal is determined by a logical AND operation ofACK/NACKs for the at least one received downlink packet.
 6. The userequipment of claim 5, wherein the processor is configured to receiveinformation about a total number of the plurality of TDD downlinksubframes from the base station.
 7. The user equipment of claim 6,wherein whether the at least one downlink packet is missed is determinedbased on the total number of the plurality of TDD downlink subframes anda number of the at least one received downlink packet
 8. The userequipment of claim 6, the processor is configured to receive an uplinkassignment for the uplink data channel from the base station.